1. Field of the Invention
The apparatus of the present invention relates generally to material transfer devices. The material transferred might include, but not be limited to, semiconductor wafers, such as silicon and gallium arsenide, semiconductor packaging substrates, such as high density interconnects, semiconductor manufacturing process imaging plates, such as masks or reticles, and large area display panels, such as active matrix LCD substrates.
2. Description of the Prior Art
Throughout this disclosure, the term "wafer" will be used for purposes of consistency to refer to such substrates as just mentioned, but it will be understood that the term is intended to be used in the broad context so as to be applicable to all substrates. Typically, such substrates are circular and have a diameter of 200 mm and thickness of approximately 0.760 mm although, more recently, the diameter of choice has evolved to 300 mm with the same thickness. The invention is especially beneficial for operating on the newer size of substrates.
The transfer of delicate silicon wafers of the like between a plurality of work stations or locations in the manufacture of semiconductor devices presents unique handling problems. The silicon wafers are very delicate and have highly polished surfaces, yet for maximum throughput must be handled rapidly and with accuracy. Previously, it was customary to place cassettes, that is, portable carriers generally employed for supporting and transporting a plurality of wafers in spaced stacked relationship manually into a load lock or other intermediate holding station. Such a holding station is proximate to a further transfer station from which wafers are typically transported to and from a plurality of processing stations. In known constructions, load locks require a human or robot loader to place wafers or cassettes holding multiple wafers into the load lock. This procedure is often difficult because of customarily small clearances between the chamber walls and the wafers.
Various constructions are known for transferring wafers from one location to another. Typical of such apparatus are the disclosures in U.S. Pat. No. 4,674,936 to Bonora, U.S. Pat. No. 4,802,809 to Bonora, and U.S. Pat. No. 5,169,272 to Bonora et al., each of which disclose apparatus for transferring a cassette supporting multiple wafers from a SMIF (standard mechanical interface) system container.
In U.S. Pat. No. 4,730,976 to Davis et al., an articulated arm assembly extends and retracts in a "froglike" motion to transfer a wafer between a plurality of locations. The articulated arm assembly is rotatable about a pivot point in a radial plane and can be raised and lowered in an axial direction.
An improvement on Davis et al. is provided in commonly assigned U.S. Pat. No. 5,180,276 to Hendrickson which again utilizes a "froglike" or "frogkick" type of motion. Another construction of wafer transport apparatus is disclosed in International Application No. PCT/US87/01176 to Westrake which discloses a mechanism for retrieving a wafer from a cassette in a load lock and advancing it into a reactive chamber for processing.
It was in light of the foregoing state of the art that the present invention has been conceived and is now reduced to practice. Specifically, the invention results from efforts to increase throughput of existing wafer handling systems in a manner compatible with existing systems while assuring protection for the wafers.